The development of a flexible fibre for CO.sub.2 laser power delivery is regarded as a prerequisute to the further diversification of this laser's applications concerning industrial and medical systems. Significant progress has been made in recent years towards the commercial realisation of such a fibre, and a number of potential materials and designs are being actively pursued.
The principal concern is the requirement to provide for high transmission at 10.6 .mu.m both in straight guide, and around curvatures of a few centimeters radius. However, other properties of the fibre material do assume particular importance and these will depend on the application envisaged. For medical systems requiring flexible delivery of CO.sub.2 laser energy, consideration on the choice of material should also take account of the associated fabrication costs, bio-compatibility (i.e. toxicity), photo-degradation, mechanical and environmental stability and also shelf-life. Such requirements tend to point towards a glass-forming material. However, as yet there does not appear to be a suitable glass which will fulfill all of these when constructed in solid core fibre form. Most glasses, in common with many other inorganic materials, exhibit strong absorption characteristics at mid infra-red frequencies due to resonance effects in the molecular structure, and are substantially opaque to such radiation.
Optical fibres in solid core form which do transmit at infra-red wavelengths have been made from certain crystalline halide materials. Examples of these are the KRS5 fibre developed by Horiba Ltd of Japan, (e.g. Ref. 1); and the silver chloride/silver bromide fibres of Katzir and Ariele (Ref. 2). However, these types of materials do pose a number of disadvantages when their mechanical and environmental properties referred to above are taken into consideration.
To address the problems associated with solid core type materials, there are a number of proposals which concern the fabrication of hollow waveguides. These include hollow metallic waveguides (Ref. 3), dielectric coated hollow metallic waveguides (Ref. 4,5), and hollow circular glass waveguides (Ref. 6). It is to hollow optical fibre waveguides of the last type which the present invention particularly relates. In this construction, the glass acts as a cladding to the fibre core which is air. Clearly to derive a condition of total internal reflection, and thus high transmission, the glass cladding must maintain a refractive index of less than unity at the wavelength of interest. Many inorganic materials, either in vitreous (i.e. amorphous) or crystalline form, exhibit strong anomalous dispersion in the mid-infrared to the extent that their refractive indices can assume values of less than unity at certain wavelengths. The physical processes governing the resonance absorption and dispersion effects are the molecular vibrational transitions of the material.
The utilisation of these effects and their wavelength turning to 10.6 .mu.m for inorganic oxide glasses was first proposed by Hidaka et al (Ref. 7) in a novel approach to the development of a flexible waveguide. The material parameter of interest is the complex refractive index of the cladding which is defined as N=n-iK, wherein n=refractive index, and K=extinction coefficient.
For the development of novel materials these optical constants need to be determined at infra-red wavelengths with reasonable accuracy and the values at 10.6 .mu.m minimised for high transmission. A zero K value will give zero attentuation of the internally reflected radiation, but in practice this is virtually impossible to achieve, since it is a relatively high value of K which gives rise to the refractive index n being less than 1. This in turn provides the totally internal reflecting properties of the fibre. For any given glass, however, the maximum value of K is at a slightly longer wavelength than the minimum value of the refractive index, and generally speaking the minimum transmission loss of a hollow fibre waveguide made from the glass will be at a wavelength slightly shorter than the minimum value of refractive index. Also, since these parameters will depend on the atomic masses and molecular structure of the glass constituents, it follows that the composition of the glass from which the fibre is made will very much affect the performance of the fibre at a given wavelength.
It has been determined that the minimum refractive index of pure germanium dioxide (GeO.sub.2) glass is very close to the CO.sub.2 laser wavelength of 10.6 .mu.m (943 cm.sup.-1). Thus, germanium dioxide based glasses are potential hollow optical fibre waveguides for good transmission in this wavelength region. Indeed a hollow optical fibre waveguide made from a glass consisting of 80 mole % germanium dioxide, 10 mole % ZnO, 10 mole % K.sub.2 O is described in the patent by T. Hidaka (Ref. 6). Similarly, EP-A-0249886 discloses a method for the manufacture of hollow glass fibres of silica which are provided with a glass cladding on their internal surface. Claddings of 80 mole % and 100 mole % GeO.sub.2 are exemplified as suitable for manufacture by the disclosed method.